1. Field of the Invention
This invention relates to an exhaust manifold.
2. Description of the Related Art
Recently, an exhaust manifold as an engine exhaust pipe has been made of stainless (SUS), in stead of conventional cast iron, to comply with requirements for exhaust gas regulations, high heat resisting properties, and reduction of weight. Further, a double tube structure having a thin thickness inner tube has been becoming a mainstream for the stainless exhaust manifold to improve warming-up property of catalysts.
An example of this kind of stainless double tube exhaust manifold is shown in FIGS. 33-35. An exhaust manifold 80 shown in FIGS. 33-35, which is mounted on a left bank (#1, #3, #5 cylinders) of a V-type six-cylinder engine, is formed from two plate members each deformed into an exhaust manifold shape and welded at joining faces. The exhaust manifold 81 has head flanges 81 that are bolted to specific portions of exhaust ports of cylinders on an engine side, and a flange 82 that is bolted to a crossover pipe. An inner tube 83 having a thickness of 0.8 mm is welded to the head flanges 81, while an outer tube 84 having a thickness in a range of 20. mm to 3.0 mm is welded to the flange 82 as well as to the head flanges 81. As shown in FIG. 35, the inner and outer tubes 83, 84 respectively have butting portions 85, 85a facing to one another on a side opposite to the head flange side, and each of the inner and outer tubes 83, 84 is welded at the respective butting portion 85 or 85a. High pressure exhaust gas discharged from the exhaust port of each cylinder passes through the exhaust manifold 80 and is discharged into atmosphere through the crossover pipe, a front pipe, and a muffler.
However, the exhaust manifold 80 having the structure described above generates high frequency noise (radiation noise) having more than 1 kHz in frequency. This problem is peculiar to the exhaust manifold made of stainless. That is, as shown in FIG. 35, pressure waves of exhaust gas compressed in the engine enter the inner tube 83 immediately after an exhaust valve is opened. At that time, the pressure waves progress toward the butting portion 85 in direction P1 while vibrating flat portions 86 of the inner tube, is reflected at the butting portion 85, and then returns in direction P2 while vibrating the flat portions 86 again. The vibration generated at the flat portions 86 is transmitted to the entire area of the inner tube 83. The vibration is further transmitted to the outer tube 84 from the inner tube 83 through the head flanges 81, and then causes the high frequency (1 kHz-20 kHz) noise.